There are pre-built floppy images on the net,
for example from freedos, you might want to
test those to see if it's a general problem
on your system (they work fine here).
The msdos floppies might use a special disk
geometry, dosbox has only support for standard
disk sizes.
Most of the time it's enough to copy all files
from ALL disks into one directory and mount
that as drive a (mount a c:\dosdisks).